Code should not be marked as unreachable in certain situations

[cperezabo]

Environment data

• VS Code version: 1.50.0
• Extension version (available under the Extensions sidebar): v2020.9.114305
• OS and version: macOS 10.15.7
• Python version (& distribution if applicable, e.g. Anaconda): CPython 3.8.2
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): Remote Container, no venv.
• Value of the python.languageServer setting: Pylance

Expected behaviour

The code should not be marked as unreachable when it is perfectly reachable

Actual behaviour

As you can see, the above code is marked as unreachable just because dangerous raises an exception.
I am justly testing that stack.capture() captures the exception. I think is not correct to mark the code as unreachable when there is a context manager that can handle the exception.

[erictraut]

A type checker needs to be told that your context manager can handle exceptions. Most of them do not.

Type checkers (including mypy and pyright/pylance) have adopted a convention whereby if the __enter__ method is annotated to return a bool, it is assumed that the context manager handles exceptions. Refer to this thread for details.

[cperezabo]

Annotate __enter__ to return a bool? Doesn't make sense but I have tried it anyways and doesn't work.
And if it were, what about @contextmanager decorator? you can't annotate __enter__

Moreover, the same code has no issues in PyCharm

[erictraut]

Sorry, the thread I referenced said __exit__, not __enter__. You're correct that this technique wouldn't work with the @contextmanager decorator. There's really no good solution here. If a static analyzer assumes that all context managers catch all exceptions, that will cause a different set of problems. Since most context managers do not catch all exceptions, it's better to assume that they don't unless informed otherwise.

Here's another simple workaround for your particular case. You can provide a None return type annotation for the dangerous method. Because your code doesn't provide a return type declaration, pylance is inferring the return type. And in this case, it's inferring a NoReturn because all of the code paths raise an exception.

def dangerous() -> None:
    raise FakeException

[cperezabo]

You are right, annotating it with None does the job!